fix problems with continuations & sharing

When continuation C2 extends continuation C1, C2 shares the copy
of the internal stack with C1. It needs to skip the bit of
C1's stack that corresponds to arguments to `call/cc`, though.
That skipping assumed that `call/cc` takes 1 argument, but it can
take 2. The bug broke `racklog`, which captures continuations using
its own prompt. (It seems like there should be a simple test that
is independent of Racklog, but I couldn't construct it.)

Meanwhile, the continuation shouldn't retain the arguments to
`call/cc`, so clear them. (That was easy to test.) Sharing still
has to compensate for the locations of the arguments, though.